1) One of your previous significant others whom you've had a relation-shit, and coincidentally, a break up with. In most cases, not someone you ever want to see again. In very VERY rare cases, someone you might still talk to and consider a friend, because you both have a history and too many experiences to forget about together and are both mature enough to push any emotions you have left (if any), aside.

2) Your current bf/gf's ex. AKA an evil being that must be destroyed, at all costs. If you're a girl- for some reason, your bf's ex always seems to be a blonde with big boobs and a shitty personality who for some reason, makes you feel fat. If you're a guy- somehow, your gf's ex is famous among every girl you know, and if you ever bring him up, they can't stop telling you how "funny" and "amazing" he is. You also can't stop yourself from wondering if his equipment's bigger than yours.

1) The last two months of me and my ex's 1-year relationship were hell due to our constant fighting over stupid shit. So we broke up and I never want to see him/her again.
-or-
My ex and I broke up after dating for two years because he/she had to move. We got along really well during those first two years, so we still keep in touch.

2) (After current gf meets ex gf)
Gf: "Well... she was pretty."
Bf: "Uhh..."
Gf: "...Does this skirt make me look fat?! She was skinnier than I am! Why'd she have to be so skinny? What is she, anorexic?!"
Bf: "You look fine. You're just feeling the jealousy and insecurity that can only be brought on by seeing your significant other's ex..."